Adds labs to walkthrough

- updates README
- minor updates to GL+ walkthrough intro

And separates the existing Gerrit provider to a new GoogleSource one.

Closes #1953

Trying to improve clarity that there are only a small subset of features that require an account

- adds headings, and moves the majority of text content to the body of each step
- most steps now have a simple, one-line description
- adds "try now" section to premium feature walkthrough with a button to try
- adds and updates content in premium feature introduction step
- removes an unnecessary file

- adds new images to premium feature sections
- fixes side bar view images to be side-by-side
- in the last step of the getting started walkthrough, add image and a button that links to the premium feature walkthrough
- updates the old settings.png image with a new version
- adds a new image of the quick setup
- adds a note about worktrees not working on the web